The use of triarylamine units as emitters, hole transporters and host materials in opto-electrical devices is known as will be evident from the following description of the background to the invention which outlines a basic device structure and some known materials for use in such a structure.
One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In WO90/13148 the organic light-emissive material is a polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminium (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent first electrode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminium, or a plurality of layers such as calcium and aluminium. Other layers can be added to the device, for example to improve charge injection from the electrodes to the electroluminescent material. For example, a hole injection layer such as poly (ethylene dioxythiophene)/polystyrene sulfonate (PEDOT-PSS) or polyaniline may be provided between the anode and the electroluminescent material. When a voltage is applied between the electrodes from a power supply one of the electrodes acts as a cathode and the other as an anode.
In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
For organic semiconductors, important characteristics are the binding energies, measured with respect to the vacuum level of the electronic energy levels, particularly the “highest occupied molecular orbital” (HOMO) and the “lowest unoccupied molecular orbital” (LUMO) level. These can be estimated from measurements of photoemission and particularly measurements of the electrochemical potentials for oxidation and reduction. It is well understood in this field that such energies are affected by a number of factors, such as the local environment near an interface, and the point on the curve (peak) from which the value is determined. Accordingly, the use of such values is indicative rather than quantitative.
The optical and electronic properties of an organic semiconductor are highly dependent on the energy of the aforementioned HOMO and LUMO levels. Furthermore, these energy levels are highly dependent on the chemical structure of the organic semiconductor. By selecting suitable materials, or combinations of materials, device performance can be improved.
For example, one way of improving efficiency of devices is to provide hole and electron transporting materials. WO 99/48610 discloses blending of hole transporting polymers, electron transporting polymers and electroluminescent polymers. A 1:1 copolymer of dioctylfluorene and triphenylamine is disclosed as a hole transporting polymer in this document. The type of charge transporting material which is most effective will be dependent on the HOMO and LUMO of the other components in the device.
Although there has been much improvement in the efficiency of devices using charge transporting materials, there is always a desire to develop new charge transporting materials to further improving efficiency when compared with existing devices.
Another focus in the field of polymer OLEDs is the development of full colour displays for which red, green and blue emissive materials are required. By “red electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 600-750 nm, preferably 600-700 nm, more preferably 610-650 nm and most preferably having an emission peak around 650-660 nm. By “green electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 510-580 nm, preferably 510-570 nm. By “blue electroluminescent material” is meant an organic material that by electroluminescence emits radiation having a wavelength in the range of 400-500 nm, more preferably 430-500 nm.
One drawback with existing polymer OLED displays relevant to this development is the relatively short lifetime of blue emissive materials known to date (by “lifetime” is meant the time for the brightness of the OLED to halve at constant current when operated under DC drive).
In one approach, the lifetime of the emissive material may be extended by optimisation of the OLED architecture; for example lifetime of the blue material may in part be dependant on the cathode being used. However, the advantage of selecting a cathode that improves blue lifetime may be offset by disadvantageous effects of the cathode on performance of red and green materials. For example, Synthetic Metals 111-112 (2000), 125-128 discloses a full colour display wherein the cathode is LiF/Ca/Al. The present inventors have found that this cathode is particularly efficacious with respect to the blue emissive material but which shows poor performance with respect to green and, especially, red emitters.
Another approach is development of novel blue electroluminescent materials. For example, WO 00/55927, which is a development of WO 99/48160, discloses a blue electroluminescent polymer of formula (a):
                wherein w+x+y=1, w≧0.5, 0≦x+y≦0.5 and n≧2.        
In essence, the separate polymers disclosed in WO 99/48160 are combined into a single molecule. The F8 repeat unit is provided for the purpose of electron injection; the TFB unit is provided for the purpose of hole transport; and the PFB repeat unit is provided as the emissive unit. The combination of units into a single polymer may be preferable to a blend. For example, intramolecular charge transport may be preferable to intermolecular charge transport. Potential difficulties of phase separation in blends is also avoided.
WO 02/92723 and WO02/92724 disclose replacement of some of the F8 repeat units in the polymer illustrated above with 9,9-diarylfluorene repeat units, in particular diphenylfluorene (DPF) repeat units which has surprisingly been found to improve lifetime of the polymer.
WO 99/54385 and EP 1229063 disclose copolymers of fluorenes and PFB-type triarylamine repeat units. EP 1229063 discloses a copolymer of F8:TFB in a 70:30 ratio.
Although there has been much improvement in the lifetime of blue emissive materials there is always a desire to develop new blue emissive materials to further improve lifetime of the polymer.
Phosphorescent materials are also useful and in some applications may be preferable to fluorescent materials. One type of phosphorescent material comprises a host and a phosphorescent emitter in the host. The emitter may be bonded to the host or provided as a separate component in a blend.
Numerous hosts for phosphorescent emitters are described in the prior art including “small molecule” hosts such as 4,4′-bis(carbazol-9-yl)biphenyl), known as CBP, and (4,4′,4″-tris(carbazol-9-yl)triphenylamine), known as TCTA, disclosed in Ikai et al. (Appl. Phys. Lett., 79 no. 2, 2001, 156); and triarylamines such as tris-4-(N-3-methylphenyl-N-phenyl)phenylamine, known as MTDATA. Homopolymers are also known as hosts, in particular poly(vinyl carbazole) disclosed in, for example, Appl. Phys. Lett. 2000, 77(15), 2280; polyfluorenes in Synth. Met. 2001, 116, 379, Phys. Rev. B 2001, 63, 235206 and Appl. Phys. Lett. 2003, 82(7), 1006; poly[4-(N-4-vinylbenzyloxyethyl, N-methylamino)-N-(2,5-di-tert-butylphenylnapthalimide] in Adv. Mater. 1999, 11(4), 285; and poly(para-phenylenes) in J. Mater. Chem. 2003, 13, 50-55.
A problem with known host-phosphor systems is that the host may quench emission from the phosphor. In general, the lower the triplet energy level of the host (relative to the phosphor) then the more likely quenching will occur. Polymerisation can exacerbate this problem by reducing the triplet energy level of a monomer when forming a host polymer. Accordingly, there is a need to produce materials with a high triplet energy level for use as hosts in phosphorescent systems.
Such host-emitter systems are not limited to phosphorescent devices. A wide range of fluorescent low molecular weight metal complexes are known and have been demonstrated in organic light emitting devices [see, e.g., Macromol. Sym. 125 (1997) 1-48, U.S. Pat. No. 5,150,006, U.S. Pat. No. 6,083,634 and U.S. Pat. No. 5,432,014].
As with phosphorescent systems, a problem with known host-fluorescent emitter systems is that the host may quench emission from the fluorescent emitter. It is advantageous to provide a host having a higher LUMO than that of the emitter to inject electrons into the emitter. It is advantageous to provide a host having a lower HOMO than that of the emitter to inject holes into the emitter. Accordingly, there is a need to produce materials with a large band gap between the HOMO and LUMO for use as hosts in fluorescent systems.
Another factor affecting the performance of opto-electonic devices is morphology of the films which make up the device. For semiconductive organic materials it is advantageous to have an amorphous rather than a crystalline film. However, it is desirable not to have too much disorder in the film in order to achieve a device with better performance. Accordingly, there is a desire to produce materials with better film forming characteristics.
Another problem with known devices results from charge migration past the emitting regions. Accordingly, it is sometimes useful to provide charge-blocking materials. There is a desire to produce materials with better charge blocking characteristics. A low HOMO may aid in blocking holes while a high LUMO may aid in blocking electrons.
It is an aim of the present invention to solve one or more of the problems outlined above.